Characteristics And Mechanism Of Transmission Of ESBL-Producing Escherichia Coli From Cattle In Guanzhong Of Shaanxi Province

Escherichia coli（E.coli）,as a pathogen of symbiosis,is closely related to the health of animals,and is also the main cause of common diseases in dairy farms,such as calf diarrhea,mastitis and endometritis.As a major user of antibiotics,China has an alarming number of antibiotics used as treatments or additives in animals.Some commonly used antibiotics,such asb-lactams,aminoglycosides and fluoroquinolones,have gradually weakened or lost their therapeutic effect.The drug resistance of animal-derived E.coli not only restricts the development of animal husbandry,but also is closely related to the human health,and can spread to humans through food chain,air,soil and water pollution.At present,the reports on the drug resistance to animal-derived E.coli have been increasing year by year and very diverse,which has attracted the attention of researchers in various countries.Extended-spectrumβ-lactamases（ESBL）are a class of enzymes capable of hydrolyzing extended-spectrum cephalosporin antibiotics.Most of the genes encoding this enzyme are carried by plasmids which is spreads at an alarming rate in humans and animals,leading to severe bacterial resistance.In this study,ESBL-producing E.coli was isolated and identified from calf feces samples in Guanzhong,Shaanxi,and its drug resistance characteristics and molecular epidemiological typing were tested,and transmission mechanism were studied.The results are as follows:1. Seventy-five strains of E.coli were isolated from 4 dairy farms in Guanzhong area,Shaanxi province.The results of the ESBL phenotype confirmed test showed that 59 strains（78.67%）produced ESBL and were all multi-drug resistant strains.The detection rates of bla _CTX-M,bla _TEM-1,bla _CMY-2and bla _DHA-1in 59 ESBL-producing strains were 100.00%,67.80%,3.39%and 1.70%,respectively.bla _CTX-M-55（n=36）is the dominant genotype.The 42.37%ESBL-producing E.coli contained 16S r RNA methylase gene,and the 18 strains carried both rmt B and bla _CTX-M-55.67.80%ESBL-producing E.coli carried at least one PMQR gene,among which qnr S（70.00%）was the most common.It is noteworthy that one strain also carried bla _CTX-M-55,bla _TEM-1,bla _CMY-2,bla _DHA-1,rmt B,qnr S,qnr B,aac（6’）-ib-cr genes.2. The most common phylogenetic group of ESBL-producing E.coli is type C,indicating that most ESBL-producing E.coli are symbiotic.Twenty-seven different ST types were obtained from 59 strains of ESBL-producing E.coli,among which ST744 was the main prevailing ST type and ST23_CCwas the main ST clone complex,The results showed that ESBL-producing E.coli had some genetic diversity in this area.3. Thirty-six strains of ESBL-producing E.coli with bla _CTX-M-55were selected for conjugation transfer,and 28 strains could transfer plasmids to the E.coli 26R793.In addition,the rmt B gene carried by 10（10/18,55.56%）strains was successfully transferred to E.coli26R793 along with the bla _CTX-M-55gene.Four different types of replicons were detected in 36strains of bla _CTX-M-55donor bacteria and 28 strains of transconjugants,including Inc FIB,Inc I1,Inc Frep B,and Inc N,among which Inc FIB type was dominant.After the acquisition of resistant genes,the bacteria can be inherited stably without affecting its growth trend.The ISEcp1 element provides a favorable condition for the spread of resistant genes between different bacteria.